The present invention relates to a method of manufacturing a spark plug and the spark plug.
As spark plugs serving as igniting means for internal combustion engines, spark plugs of a type having a noble-metal igniting portion have been used in recent years to improve resistance against sparks, the spark plugs being formed at the leading end of an electrode by welding a noble metal chip, the main component of which is Pt or Ir. When a noble metal chip is joined to the leading end surface of, for example, a central electrode, a method has been disclosed for example, in JP-A-6-45050 (U.S. Pat. No. 5,320,569, EP-0 583 103 B1) and JP-A-10-112374. The method has the steps of superimposing a disc-shaped metal chip on the leading end of the central electrode and irradiating the outer surface of the superimposed surfaces with a laser beam while the central electrode is being rotated so that a perimeter laser weld portion is formed.
In recent years, the temperature in the combustion chamber has been raised because the performance of the internal combustion engine has been improved. Moreover, engines of a type having a structure that the diameter of the igniting portion of the spark plug is reduced to be 2.0 mm or smaller so that the igniting portion is allowed to project into the combustion chamber in order to improve ignition easiness have been widely employed. To improve resistance of the noble metal chip against separation in the severe atmosphere for use, a method has been employed with which a noble metal chip, the main component of which is Ir or Pt, is welded to an electrode made of a heat resisting alloy composed of Ni groups or Fe groups so as to form a noble-metal igniting portion.
The noble metal chip is usually welded to the spark plug by using pulse laser beams, such as YAG laser beams. The conditions under which the laser beam is applied have been determined such that the pulse generating frequency is a relatively low value of 0.5 pulse/second or lower. The foregoing method, however, requires an excessively long time of about 15 seconds to weld a noble metal chip having a diameter of about 0.7 mm. Therefore, there arises a problem of unsatisfactory productivity for each laser welding apparatus.
To solve the problem of deterioration in the productivity, it is effective to raise the pulse generating frequency of the laser beams. As a result of investigations performed by the inventor of the present invention, the following problem has been found. When the pulse generating frequency of the laser beam is simply raised to improve the productivity, the heat resisting alloy composed of Ni groups or Fe groups for use as the material of the electrode and having a low heat conductivity suffers from an unsatisfactory fall in the temperature. When the leading end of the electrode has a small diameter, the fall in the temperature of the electrode is sometimes delayed as compared with application of heat which is performed for each pulse of the laser beam. As a result, the rear portions of a weld portion 10 in the circumferential direction are heated excessively, as shown in FIGS. 13A and 13B. Therefore, there arises a problem in that the weld portion is made to be nonuniform such that a rear weld portion 10s is deeply welded as compared with a front weld portion 10p. As an alternative to this, the width I of the weld portion is enlarged excessively. Since the weld portion 10 is constituted by an alloy of a material of a noble metal chip and a material of the electrode, the resistance against sparks of the weld portion 10 is inferior to that of only the noble metal chip. If the depth or the width of the weld portion 10 is enlarged excessively, the durability of an igniting portion 31 formed by welding the noble metal chip excessively deteriorates. In a region in which the width I of the weld portion 10 has been enlarged, the weld portion 10 is exposed to a discharging surface 31a in extreme cases. If the exposure is prevented, exposure of the weld portion 10 occurs in a case where the igniting portion 31 is somewhat consumed. In general, a spark plug is provided with the noble-metal igniting portion for the purpose of elongating the lifetime of the igniting portion (for example, lifetime corresponding to driving for 100,000 km to 160,000 km). If the foregoing problem arises, consumption of the exposed portion proceed. As a result, there sometimes arises a problem of misfire or the like because the spark discharge gap is enlarged in a relatively short time.
An object of the present invention is to provide a method of manufacturing a spark plug capable of significantly improving efficiency of a process for welding a noble metal chip to an electrode and obtaining a uniform weld portion and a spark plug having durability of an igniting portion to a degree not heretofore attained.
To solve the problems, according to the present invention, there is provided a method of manufacturing a spark plug comprising a central electrode and a ground electrode disposed such that the side surface of the ground electrode is opposite to the leading end surface of the central electrode and having a structure that a noble-metal igniting portion having a discharge surface is provided for at least either of the central electrode or the ground electrode at a position corresponding to a spark discharge gap by welding a noble metal chip, the method of manufacturing a spark plug comprising the steps of:
constituting at least a chip securing surface forming portion of the central electrode and/or the ground electrode is formed with a heat resisting alloy having Ni or Fe as a main component thereof and superimposing the noble metal chip on a chip securing surface so as to form a superimposed assembly;
providing a perimeter laser weld portion across the noble metal chip and the chip securing surface forming portion for the outer surface of the chip constituting the superimposed assembly so that the noble metal chip is secured to the chip securing surface; and
forming the perimeter laser weld portion having a maximum outer dimension dmax plane-viewed from the chip interposing direction is smaller than 2.0 mm and so formed not to reach the discharge surface in a direction of the thickness of the noble metal chip by using, as a light source for laser welding, a pulse laser beam source having energy per pulse of 1.5 J to 6 J, a pulse length of 1 millisecond to 10 millisecond and a pulse generation frequency is 2 pulse/second to 20 pulse/second.
The chip securing surface forming portion is constituted by the heat resisting alloy, the main component of which is Fe or Ni, and the small-diameter perimeter laser weld portion having a maximum outer dimension dmax which is smaller than 2.0 mm is formed. The inventors of the present invention has energetically performed studies. Thus, the following facts have been detected. That is, it is important that the pulse generating frequency satisfies a specific range to efficiently form a uniform weld portion. Moreover, it is important that conditions of the energy per pulse of a laser beam and the length of each pulse are made to be specific values. The following fact has been detected: in a state where the foregoing conditions are set, the problem of the nonuniform weld portion and the like can be solved if a frequency higher than the frequency employed in the conventional method is employed in only a specific pulse frequency range. Thus, the present invention has been established. That is, the laser beam having energy per pulse of 1.5 J to 6 J, a pulse length of 1 millisecond to 10 milliseconds is employed. Moreover, 2 pulse/second to 20 pulse/second which is a pulse generating frequency which is considerably higher than the pulse generating frequency employed in the conventional method is employed. Thus, a perimeter laser weld portion exhibiting excellent uniformity can significantly efficiently be formed.
In this specification, the maximum outer dimension dmax of the perimeter laser weld portion is, as shown in FIG. 2B, defined as dmax=2 rmax when the position of the geometric center of gravity of the discharging surface (31a) realized when the perimeter laser weld portion is projected onto a plane which is perpendicular to the central axis of the central electrode (3) is G and the distance from G to a farthermost end of the perimeter laser weld portion is rmax.
When the length of the pulse is shorter than 1 millisecond or when the energy of each pulse is smaller than 1.5 J, the quantity of heat which is input per pulse is reduced and, therefore, the weld portion cannot satisfactorily be formed. When the energy of each pulse is smaller than 1.5 J and also the length of the pulse is shorter than 1 millisecond, the quantity of heat input is reduced excessively. When the electrode is constituted by the heat resisting alloy composed of the Ni groups or the Fe groups, the influence of fall of the temperature of the electrode causes a fact that the electrode is not substantially fused. In the foregoing case, the weld portion cannot easily be formed.
When energy of each pulse is larger than 6 J or when the length of the pulse is longer than 10 milliseconds, accumulation of input heat caused from the laser beams easily occurs because the fall of the temperature of the electrode constituted by the heat resisting alloy composed of the Ni groups or the Fe groups is very small. Thus, there is apprehension that nonuniformity of the depth or the width of the weld portion occurs or the electrode is melted and deformed. When the energy of each pulse is larger than 6 J and the length of the pulse is longer than 10 milliseconds, molten metal is easily evaporated and scattered. In the foregoing case, there is apprehension that defects of the electrode, such as dents and holes, easily occur.
When the pulse generating frequency is lower than 2 pulse/second, the weld portion cannot efficiently be formed. When the pulse generating frequency is higher than 20 pulse/second, accumulation of heat inputs caused from the laser beam easily occurs. In the foregoing case, similar problems arise. It is preferable that the energy of each pulse is 2 J to 5 J. It is preferable that the length of the pulse is 1.5 millisecond to 6 milliseconds. It is preferable that the pulse generating frequency is 2 pulse/second to 12 pulse/second. In this specification, energy of each pulse is a value obtained from a process in which a laser beam emitted from the laser beam source is received by an energy detection apparatus, such as a calorimeter or a power meter, before the laser welding operation is performed. Thus, energy per unit time (for example, one second) is measured, and then the energy is divided by the number of pulses per second.
When the foregoing method is employed, the perimeter laser weld portion having the maximum outer dimension dmax which is smaller than 2.0 mm is formed by using the heat resisting alloy composed of Fe groups or Ni groups as follows: A heretofore difficult laser weld portion can be formed. Specifically, a laser weld portion can be formed which has a structure that a ratio lmin/lmax of a minimum width lmin and a maximum width lmax of the perimeter laser weld portion in the superimposing direction of the noble metal chip is 0.7 or higher.
A spark plug according to the present invention comprises: a central electrode; a ground electrode disposed such that the side surface of the ground electrode is opposite to the leading end surface of the central electrode; and a noble-metal igniting portion having a discharge surface and provided for at least either of the central electrode or the ground electrode at a position corresponding to a spark discharge gap by welding a noble metal chip. In the spark plug, a chip securing surface forming portion of the central electrode and/or the ground electrode is constituted by a heat resisting alloy having Ni or Fe as a main component thereof. A perimeter laser weld portion is formed on the outer surface of the chip across the noble metal chip and the chip securing surface forming portion. A maximum outer dimension dmax of the perimeter laser weld portion plane-viewed from the chip interposing direction is smaller than 2.0 mm. The perimeter laser weld portion does not reach the discharge surface in the thickness direction of the noble metal chip. A ratio lmin/lmax of the perimeter laser weld portion in the superimposed direction to the chip securing surface is 0.7 or higher.
The fact that lmin/lmax can be made to be 0.7 or higher means a fact that circumferential dispersion of the distance from the discharging surface of the noble-metal igniting portion to the end of the perimeter laser weld portion adjacent to the discharging surface can satisfactorily be prevented. As a result, a problem can effectively be prevented which arises in that the discharging surface of the weld portion is exposed to the outside at a position at which the width of the laser weld portion is enlarged and the durability of the spark plug deteriorates. The manufacturing method according to the present invention is able to make the ratio lmin/lmax to be 0.9 or higher which is furthermore preferred value by appropriately determining the welding conditions.